The present invention generally relates to a method for flameless atomic absorption spectroscopy for introducing a sample into a graphite tube which is held at its ends by contact pieces and, in particular, relates to a method in which the sample is applied to a sample carrier which is moved into a first position outside the graphite tube where the sample is heated for ashing and thereafter the sample carrier is moved into a second position inside the graphite tube where the sample is atomized.
In a graphite tube atomizer a graphite tube is positioned between two annular contact pieces. Through the contact pieces, which are usually designed as cooling chambers flown through by a coolant, electric current can be passed through the graphite tube thereby heating it to high temperatures. In known graphite tubes, a lateral sample introducing aperture is provided on the sleeve surface, through which aperture a liquid sample can be introduced into the graphite tube. The graphite tube is then heated in several steps to dry the introduced sample, to ash it and finally to atomize it such that a "cloud of atoms" is formed within the graphite tube, in which "cloud of atoms" the different components of the sample are present in atomic state. The measuring light beam of an atomic absorption spectrophotometer passes longitudinally through the apertures of the annular contact pieces and through the longitudinal bore of the graphite tube. This measuring light beam is formed by a radiation generated, for example, by means of a hollow cathode lamp and which only contains the resonance lines of a sought element. Ideally, the measuring light beam is absorbed only by the atoms of the sought element in the cloud of atoms such that the attenuation of the measuring light beam is a measure of the quantity of the sought element in the sample.
Usually, a sample is present in liquid state as a solution. To prevent the measurement from being affected by the solvent and to ensure a rapid atomization of the sample for the measurement, the sample is dried at low temperature during which the solvent is vaporized. The drying is followed by "ashing" during which step the sample is thermally decomposed at a higher temperature. The ashing step may generate soot which is formed by non-vaporized components of the sample, which soot subsequently falsifies the measurement by non-specific absorption of the measuring light beam. These interfering components are usually removed before the measurement proper by means of an inert gas flow which passes through the graphite tube and prevents air from entering thereinto and thus prevents the graphite tube from burning.
In known graphite tubes of this type, the sample is injected such that it collects in about the middle of the graphite tube on the lower portion of the inner wall. The temperature of the graphite tube is usually varied for drying, ashing and atomizing according to a pre-determined program.
Drying and ashing ordinarily take place inside the graphite tube, which is a straight tube, in the path of rays of the measuring light beam. Interfering components from the drying and ashing procedures, which components are not completely removed the graphite tube by the inert gas flow, may condense on the inner wall of the graphite tube and falsify the measurement.
In some cases, the atomizing temperature at which a sought element in the sample is atomized depends on the type of compound in which the element is present in the dried and ashed sample. If, then, the graphite tube is continuously heated after the ashing, it may happen that a sought element is first atomized out of one compound and then atomized out of another compound at a higher temperature. This leads to corresponding signals at the detector such that the unambiguity of the relation between peak height of the detector signal and quantity of the sought element is impaired.
For this reason, it is known to apply drops of the sample solution to a sample carrier, for example, a wire helix made of tungsten. The sample carrier with the sample solution is then moved in front of a lateral sample introduction aperture of the graphite tube. An inert gas flow is passed through the graphite tube from the ends and exits at the introduction aperture. If the graphite tube is heated, the exiting inert gas flow is also heated. The sample is thus dried by means of this hot inert gas flow and the vaporizing solvent does not enter the graphite tube. (Analytical Chemistry, Volume 51 (1979), 2375-2378).
When the sample carrier is moved closer to the graphite tube, the temperature of the sample carrier is further increased due to the heat transfer from the graphite tube, such that the dried sample is further heated and is thermally decomposed. This also occurs outside the graphite tube. Subsequently, the graphite tube is heated to the atomizing temperature. After this temperature has been reached, the sample carrier is quickly introduced into the graphite tube.
Interfering components from the drying and ashing procedure are thus prevented from condensing on the inner wall of the graphite tube. The dried and decomposed sample is heated to the atomization temperature in one step by inserting the sample carrier such that the atoms of the sought element get into the cloud of atoms simultaneously, independent of their chemical compound.
From West German OS No. 30 08 938 a graphite tube for flameless atomic absorption spectroscopy having a tubular graphite body is known which provides a lateral introduction aperture for introducing a sample and on which a tubular lateral projection is provided, which projection surrounds the introduction aperture. A drop of the sample solution is applied to a wire helix carrier. The carrier is moved in front of the introduction aperture of the graphite tube, such that with the graphite tube heated the sample is dried by means of the exiting flow of hot inert gas. Subsequently, the carrier is introduced into the tubular lateral projection of the graphite tube for thermal decomposition, and the graphite tube is then heated to the atomizing temperature.
From West German OS No. 30 09 784 a device for introducing a sample into a graphite tube in flameless atomic absorption spectroscopy having a sample carrier of electrically conducting material designed as to be introduced into the graphite tube is known in which device the sample carrier is provided with electric connections and controllably heatable outside the graphite tube by passing an electric current therethrough. The drying and ashing of the sample outside the graphite tube is effected by controllably heating the sample carrier. The sample carrier is moved into a lateral aperture of the graphite tube for atomization.
From West German OS No. 27 10 861 a device for introducing a sample into a graphite tube in flameless atomic absorption spectroscopy is known, in which a liquid sample is applied to the sample carrier and is dried and then ashed by being heated before the sample carrier is introduced into the graphite tube. This device comprises a sample carrier in the form of a straight wire made of electrically conducting material, onto which spaced helical parts are threaded. The liquid sample is applied to these helical parts. The sample carrier is arranged to be introduced into the graphite tube, the straight wire being passed through aligned lateral apertures of the graphite tube, which wire thus transversely passes through the graphite tube. The wire is provided with electric connections and is heatable thereby. A pair of parallel legs are connected to the sample carrier and hold it. The legs are mounted on a carriage which is movable transversely to the longitudinal axis of the graphite tube. These legs extend transversely to the direction of movement of the carriage on both sides of the graphite tube and maintain the straight wire, which is transversely passed through the graphite tube between the legs. By stepwise advancing of the carriage, the wire is thus drawn stepwise through the graphite tube. In that way, consecutively the different helical parts having the samples supplied thereto get into graphite tube. In passing electric current through the straight wire, the sample liquids are dried and ashed outside the graphite tube. In this way, a plurality of samples may be analyzed quickly one after the other.
The known structure is designed for the quick analysis of a plurality of samples one after the other. However, it presents various problems.
The graphite tube atomizer must be provided with two aligned, transverse bores through which the wire is passed. That is, on opposite sides of the graphite tube, two relatively large apertures have to be provided. This accelerates the dissipation of the "cloud of atoms" formed inside the graphite tube.
A graphite tube is a component to be used up which is replaced after a certain number of analyses. As the wire is passed through the graphite tube with each replacement of the graphite tube, the wire has to be detached from the legs and the electric connections and, after the old graphite tube has been removed, has to be passed through the new graphite tube. This is a tedious and a time consuming procedure and which leads to quick wear of the device.
From the West German OS No. 2 219 190 a device for atomizing a sample for flameless atomic absorption spectroscopy is known, in which the sample is introduced into a small crucible of electrically conducting material. This crucible is held by two legs which are disposed close to but spaced from each other. It is heated to atomizing temperature by passing electric current through the crucible through the legs. The crucible is surrounded by a heated tubular retainer. In contrast to more conventional graphite tubes, the only function of the retainer is to keep together the "cloud of atoms" formed by the atomizing of the sample. The heating of the retainer is not to atomize the sample, but just to prevent a condensation of the sample atoms on the retainer. OS No. 2 219 190 also suggests moving the crucible out of the cavity defined by the retainer for carrying out a drying and ashing procedure.
From the West German OS No. 29 45 646 a method for applying solid or semi-solid samples to a graphite tube atomizer for flameless atomic absorption spectroscopy is known, in which method the sample is taken up by means of a tool the lateral dimensions of which permit an introduction of the tool into the dosing aperture which is usually provided on the sleeve surface of the graphite tube. The tool together with the sample is introduced into the graphite tube through this dosing aperture. The tool may comprise a kind of spoon which is formed to be rotated through 180.degree. to spill a solid sample introduced within the graphite tube.
From the West German OS No. 2 023 336, there is furthermore known to tip aside a graphite tube atomizer having housing, contact pieces and graphite tube relative to the measuring light beam such that the graphite tube is accessible from one end through a contact piece. A solid sample may then be introduced from the end face of the graphite tube by means of a suitable tool.